In-situ structural evolution during uniaxial extension and subsequent retraction of a thermoplastic elastomer (TPE) based on propylene-dominant ethylene-propylene (EP) copolymer was studied. Combined measurements of time-resolved wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) as well as stress-strain curves revealed molecular mechanism responsible for the elastic behavior. During the first cycle of deformation, a fraction of the crystals was destroyed, while the rest was reoriented. At strains larger than 1.0, strain-induced {alpha}-crystals in the lamellar form took place, resulting in the creation of a network with well-oriented lamellae having their normals parallel to the stretching direction. With the increasemore » of strain, more crystals were induced, forming an enhanced network with strain-hardening behavior. During retraction and even after complete relaxation to zero stress, the majority of the strain-induced crystalline network remains in tact as being 'permanent set', where lamellar stacks act as the network points. This strain-induced crystalline network structure is thermally stable at room temperature and is responsible for the elastic behavior during subsequent cyclic deformation, similar to a vulcanized rubber.« less

In situ strain-induced structure changes during uniaxial deformation of an ethylene-propylene copolymer, containing 78 wt% (or 85 mol%) of ethylene moiety, were studied by synchrotron wide-angle X-ray diffraction (WAXD). The chosen sample could crystallize into orthorhombic, pseudo hexagonal or a mixed form, depending on the annealing conditions. Crystallization at high temperatures (e.g. 50 degC) favored the formation of orthorhombic form, while crystallization at low temperatures (e.g. 20 degC) favored the formation of pseudo hexagonal form. Under deformation, the transition from orthorhombic to pseudo hexagonal form was observed at relatively low strains (e.g. 0.12). At higher strains, WAXD data indicated themore » occurrence of direction-dependent crystal destruction at strains <0.25 and subsequent re-crystallization with extended-chain conformation at high strains (>1.0) all of the pseudo hexagonal form. The drastic changes in the crystalline structures (orthorhombic to pseudo hexagonal) and phase transitions (crystal destruction and re-crystallization) at modest strains can be attributed to the high mobility of the amorphous ethylene-propylene segments at room temperature.« less